In cheese technology, diffusion phenomena are crucial during brining and ripening. The FRAP technique was applied for the first time on real cheeses obtained from different technologies, in order to investigate the relationships between cheese composition and diffusion rates, using dextran molecules in a wide range of sizes. In the soft cheese, diffusion was explained by the obstruction model, meaning that the largest solutes were not more hindered by the matrix structure than the smallest ones. In the hard cheese, the obstruction model was valid only for dextrans smaller than 250 kDa, meaning that pore sizes of the matrix are probably of the same order of magnitude as the largest dextran (~30nm). Moreover, diffusion coefficients of all dextrans were linearly correlated to the “free” water content of the fat-free cheeses. FRAP is a very powerful technique to explore both diffusion properties and microstructure of complex media such as cheese.